Saris has finally extended its range of power-measuring hubs to mountain bikers with the introduction of the PowerTap SL 2.4 disc brake hub. The new model uses the same accurate internals and easy-to-use computer head as the standard SL 2.4 but includes a mountain bike-specific aluminium-and-carbon hub shell with fittings for the included proprietary 160mm rotor. Wireless 2.4GHz transmission means that there also aren't any wires to snag or route along complicated rear suspension linkages.
A wireless heart rate monitor chest strap is included in the package and an optional wireless speed and cadence sensor means you can also use the PowerTap head to display non-power related features if you prefer to use lighter wheels for race day. Speaking of which, Saris even includes shims so you can match the rotor spacing to your race wheels to ease the swap.
The as-installed weight of 734g (including hub, rotor, computer head and all mounts and hardware) isn't light but the added mass likely won't mean much to riders who are mainly interested in obtaining a reliable power measurement for training purposes.